The present invention relates to adaptive antenna systems.
Antennas are used to receive signals for imaging, communications and other purposes. The ability to detect a signal is adversely affected by the presence of extraneous transmissions such as background noise, transmissions intended for other devices, or jamming signals designed to impair the performance of the given antenna.
Adaptive antennas employ feedback to enhance their signal-to-noise ratio, or signal-to-jamming ratio. Adaptive antennas typically divide the signal received by each antenna element into a signal to be processed and directed along a main channel and an unprocessed signal to be correlated with the processed signal.
The processing normally involves weighting amplitude and phase of the signal according to an algorithm incorporating the correlation results of the divided signals. The processing further involves summing the weighted signals from plural antenna elements. A correlator, which may be a multiplier coupled with a low pass filter, correlates the processed and unprocessed signals. The correlator resultant is transformed in accordance with the predetermined algorithm to determine the weighting of subsequent signals.
Two problems arise in connection with the characterized adaptive antenna. In the first place, there are many circumstances where improved performance would be required. In the second place, where the signals involve large dynamic ranges, the limits of correlator design are quickly realized.
Dynamic range requirements of an adaptive processor are of great importance in the design of the correlator. The design requirement of the correlator is determined by the dynamic range of the output, which may equal the sum of the dynamic ranges of the two inputs. Where the dynamic ranges of the inputs are 40 dB each, the output may be 80 dB. While a 60 dB dynamic range is manageable, to achieve an 80 dB dynamic range of the correlator output is very difficult. Accommodating the increased dynamic range presents a difficult problem due to the constraints of existing component technology.
Therefore, it is common practice to insert limiters along the paths of the unprocessed signals between antenna sensors and correlators. Limiting the dynamic range of the corresponding correlator inputs reduces design requirements to readily achievable levels.
It is generally agreed that such limiting does not seriously degrade the performance of the incorporating adaptive antenna in the presence of single interferers or multiple interferers with comparable power levels. On the other hand, limiting the unprocessed signals does degrade the performance of an adaptive antenna in the presence of two or more interferers of very different power levels.
What is needed is an adaptive antenna which is effective in the presence of multiple interferers of very different power levels where large dynamic ranges are involved. Generally, improved overall performance and mitigated demands on correlator design are also desired.